Eris Trivia
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50 free Eris trivia questions with answers — science & nature quiz, new questions added Aug 2026.
This Eris trivia quiz covers the distant dwarf planet whose discovery cost Pluto its planethood. It starts with the find: Mike Brown's Palomar team, images from 2003 that software had ignored because Eris moved too slowly, NASA's 'tenth planet' announcement, the Xena and Gabrielle nicknames, and the long wait for a real name while the IAU decided what a planet was. Then the naming itself: Persephone rejected, 'Lila' abandoned, the goddess of strife adopted, and a moon called lawlessness with an accidental nod to Lucy Lawless. The science section covers Eris's 27% mass advantage over Pluto and its slightly smaller diameter, the 2010 stellar occultation, the 558-year orbit tilted 44 degrees and stretching to 98 AU, the scattered disc, its brilliant white methane frost, a day locked to Dysnomia's 15.8-day orbit, hints of a buried ocean and the New Horizons glimpse from inside the Kuiper belt. It ends with the myth: the uninvited goddess, the golden apple 'for the fairest' and the Judgement of Paris that started the Trojan War. Difficulty runs from easy to expert and is shown on each question, so it suits a classroom, a pub quiz or a space enthusiast. Every answer was checked against a reference page and each question carries its citation. Try our Pluto, Planets and Solar System quizzes for more.
30 of 50 questions with answers and explanations. Play the quiz
Q 01Eris holds which record among the known dwarf planets?
Most massive
Most massive. Eris outweighs Pluto by about 27%, yet the two are almost exactly the same width, so Eris must be much the denser world.
Q 02Which asteroid-belt world was named a dwarf planet alongside Eris and Pluto in 2006?
Ceres
Ceres. It had been here before: announced as a new planet in 1801, it was demoted to an asteroid as similar bodies kept turning up, then promoted again in 2006.
Q 03Which distant object's discovery prompted the re-check of old images that revealed Eris?
Sedna
Sedna. Finding a world that far out convinced the team that others might be hiding in pictures they had already taken, and a second look turned up Eris on frames from October 2003.
Q 04Which observatory's Samuel Oschin telescope took the images in which Eris was found?
Palomar
Palomar. Its survey photographed each patch of sky three times over three hours, and anything that shifted against the background stars was flagged as a possible new world.
Q 05Why did search software miss Eris in the 2003 images?
It moved too slowly across the sky
It moved too slowly across the sky. To cut false alarms, the software ignored anything drifting under 1.5 arcseconds an hour; Eris surfaced when the team re-ran the search with a lower limit and checked the candidates by eye.
Q 06When Eris was announced in 2005, NASA's press release hailed it as what?
The tenth planet
The tenth planet. Eris then looked bigger than Pluto, and the claim forced astronomers to define 'planet' for the first time; the vote that followed demoted Pluto instead.
Q 07Which test in the 2006 definition of a planet do Eris and Pluto fail?
Clearing its orbital neighbourhood
Clearing its orbital neighbourhood. A planet must dominate its zone, but Eris and Pluto share theirs with swarms of other icy bodies; they pass the other two tests, orbiting the Sun and being round.
Q 08Which TV series inspired the nickname Eris carried before it was officially named?
Xena: Warrior Princess
Xena: Warrior Princess. The nickname stuck for nearly two years, and an echo survives: Eris's moon is named after the Greek spirit of lawlessness, and Xena was played by Lucy Lawless.
Q 09Eris is named after the Greek goddess of what?
Strife and discord
Strife and discord. The discoverers found it fitting: the object had set astronomers quarrelling over what counts as a planet, a row that ended with Pluto's demotion.
Q 10In 2008, astronomers coined which name for dwarf planets beyond Neptune, such as Eris?
Plutoids
Plutoids. Only two worlds qualified at first, Pluto and Eris: the term covers dwarf planets orbiting beyond Neptune, so the lone dwarf planet of the asteroid belt was left out.
Q 11Which name for Eris topped a New Scientist readers' poll but was ruled out because an asteroid already had it?
Persephone
Persephone. In myth she is the wife of Pluto's Greek counterpart, which made her the readers' favourite, but asteroid 399 Persephone had held the name since 1895.
Q 12Mike Brown filed his first Eris web page under 'planetlila', an echo of which family member's name?
His daughter
His daughter. Lilah was born in 2005 as the team raced to write up the discovery, and Brown is said to have favoured Lila, a Hindu idea of the cosmos as play, as the planet's name.
Q 13What is the name of Eris's moon, discovered in 2005?
Dysnomia
Dysnomia. In Greek myth she is a daughter of the goddess Eris; the discoverers admit the name is a mouthful and simply call the moon 'Dy'.
Q 21About how long does Eris take to orbit the Sun?
560 years
560 years, roughly. Its path carries it out to some 97 times Earth's distance from the Sun, so a single lap takes more than twice as long as Pluto's.
Q 22On average, about how long does sunlight take to reach Eris?
About 9 hours
About 9 hours. Sunlight reaches Earth in about eight minutes, but Eris orbits some 68 times farther out on average, and nearly half as far again at its most distant.
Q 23Astronomers describe Eris's orbit as tilted about 44 degrees to which reference plane?
The ecliptic
The ecliptic. That is the plane the planets keep close to, and so large a world on so steep a tilt hints that bodies like it formed nearer the Sun than where they are found today.
Q 14How do astronomers think Eris's moon most likely formed?
From debris of a giant impact
From debris of a giant impact. The moon's neat, circular orbit is just what a collision would leave behind, and the same kind of smash-up is thought to have made Earth's own Moon.
Q 15A rival claim by astronomers in which country pushed Mike Brown's team to announce Eris in a hurry?
Spain
Spain. A Spanish team announced another large world that Brown's group had been quietly tracking, and the group, fearing its remaining finds were exposed too, unveiled Eris within two days.
Q 16What did the discovery of Eris's moon let astronomers calculate for the first time?
The dwarf planet's mass
The dwarf planet's mass. A moon's orbit shows how hard its parent pulls on it, so timing the moon's 16-day circuit let astronomers weigh Eris.
Q 17Eris's density, about 2.5 times water's, suggests it is mostly what?
Rock
Rock. For all its icy surface, Eris is thought to be mostly rock inside, with the ice only a relatively thin outer shell.
Q 18At about 2,300 km across, Eris is closest in size to which of these worlds?
The Moon
The Moon. Early estimates made Eris a quarter larger than Pluto, but better measurements shrank it to about two-thirds of our Moon's width.
Q 19How was Eris's true size finally pinned down in 2010?
By watching it pass in front of a star
By watching it pass in front of a star. Of the many observatories lined up along the predicted track, only three small telescopes at two sites in Chile caught the star blinking out.
Q 20Eris's entire surface covers about the same area as which country?
Russia
Russia. Unrolled flat, the whole world would cover about 17 million square kilometres: a single country's worth, on a body once thought to be bigger than Pluto.
Q 24Eris belongs to which population of trans-Neptunian objects?
The scattered disc
The scattered disc. Eris's long, lopsided orbit is typical of that far-flung population in every respect but one: few of its members are tilted anything like as steeply.
Q 25Which spacecraft has travelled farther from the Sun than Eris, now about 96 times Earth's distance?
Voyager 1
Voyager 1. Eris is far but not out of reach: the probe, launched in 1977, sailed past Eris's present distance on its way out of the Sun's protective bubble, which it left in 2012.
Q 26What is unusual about Eris's surface for a distant icy world?
It is almost pure white and uniform
It is almost pure white and uniform. That came as a surprise: Pluto, its near twin in size and surface chemistry, is blotchy and distinctly red.
Q 27Which ice did spectroscopy at Gemini North reveal on Eris's surface in 2005?
Methane
Methane. On Earth it is natural gas; on Eris it lies frozen solid, and its signature in the reflected sunlight made the new world look remarkably like Pluto.
Q 28When Eris swings closest to the Sun, what are its surface ices expected to turn into?
A thin atmosphere
A thin atmosphere. Even at its warmest Eris reaches only about -217 °C, yet that is enough for its most volatile ices to turn to gas, before freezing back onto the ground as it retreats.
Q 29Astronomers confirmed in 2023 that Eris's rotation is locked to what?
Its moon's orbit
Its moon's orbit. Usually the moon is the one that gets locked, as Earth's is; here the dwarf planet itself turns just once every 15.8 days, in step with its moon.
Q 30Eris's discoverer Mike Brown later argued that which undiscovered world lurks far beyond Neptune?
Planet Nine
Planet Nine. In 2016 Brown and Konstantin Batygin argued that an unseen giant is herding the orbits of distant icy bodies, an ironic sequel for the astronomer known as the man who killed Pluto.